The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. The system and method relate in general to oilfield equipment such as, but not limited to, offshore platforms and oilfield support vessels, such as well stimulation vessels and equipment.
Cement slurries are pumped between a casing and the earth during the construction of a wellbore to, for example, ensure zonal isolation between geologic formations. The slurry should be mixed at a precise proportion of dry cement blend and mix fluid to achieve the desired density. Once it is fully blended, the slurry is pumped at a pre-determined rate into the wellbore. In order for the cementing operation to be successful, the density and downhole rates must be accurately maintained throughout the job. Failure to operate within these job parameters may necessitate further remedial cementing operations or can even result in the complete loss of the well.
A continuous mixing process typically includes a container or “mix tub” into which mixed cement slurry is introduced, and from which that slurry is pumped downhole. Since variations in slurry density commonly occur during continuous mixing, the mix tub serves to homogenize the slurry and stabilize the slurry density before it is sent to the downhole pumps. A well-distributed velocity profile within the tub volume assists with homogenization.
The mix tub also acts as a reservoir that buffers the downhole pumping rate from variability in the cement slurry mixing rate. Since the volume of slurry in the tub may vary when there are interruptions in the slurry mixing rate, the mix tub must be effective over a wide range of resident slurry volumes. Only the portion of slurry in the mix tub that is actively circulating within the tub contributes to volumetric averaging of slurry properties. Dead volumes, such as non-circulating or slow circulating volumes, reduce the effective averaging volume of the tub.
It is desirable to minimize the amount of residual cement slurry remaining in the mix tub after the cementing operation has been completed. Since cement slurry is a solid suspension, sludge residues are common in regions of low velocity, called “dead spots.” Cement residue may tend to build up over time inside the mix tub, occasionally becoming so thick that it interferes with the cement mixing system when it hardens over time and then breaks free. Therefore, it is desirable to maintain high slurry velocities near all wetted surfaces to reduce the buildup of sludge residues. If a strong velocity gradient occurs near wetted surfaces at all times, then the tub can be said to be intrinsically self-cleaning, as residue buildup will be severely limited.
The mix tub must not cause the slurry to ingest air, which can reduce the slurry density and can have adverse effects on the pumping system. Strong inward flows from the free surface, such as is caused by the common practice of injecting the incoming slurry jet into the tub from above, entrains air and draws it into the resident slurry volume.
Since continuous mixing is a process dependent on the rates of delivery of the mixture constituents, the level of fluid in the mix tub will vary whenever there are disturbances in the incoming cement and fluid flows or when there are changes in the discharge slurry rate. The mix tub is also pumped empty at the end of a cement job, as well as between phases requiring different slurry formulations. Therefore, the mix tub must perform all of its principal functions over a wide range of fluid levels.
The above features in existing mix tubs are typically provided through the use of an agitation device, baffles, and/or extra volume. With an agitation device, a mechanical agitation device (such as a paddle mixer) is used to induce fluid movement within the tub. The effectiveness in homogenization diminishes with distance from the agitation device, which means that high fluid velocities are not maintained on all wetted surfaces within the tub. This approach is also undesirable, because it adds cost and complexity to the system. Baffles are used in the tub to enforce a certain pattern of fluid flow. This approach is undesirable, because a baffle always creates a dead spot upstream that encourages separation, sedimentation and accelerates cement build-up. When only part of the tub volume is activated into circulation, the overall tub volume is typically increased to compensate for dead volumes in order to maintain the true averaged volume. A common solution to this problem is to add a separate averaging tub with a volume that may be two to three times that of the mix tub in order to increase the effective averaging volume. This extra averaging tub disadvantageously increases the size of the system and adds complexity to the cementing system.
It is desirable, therefore, to use a mix tub or container that provides a desired slurry behavior in the tub without requiring baffles or a supplemental agitation device.